Abstract:An ultrastructural study of the cellular events during the early stages of organization in the zones of localized active cell division was made in the callus cultures of Stevia rebaudiana.The callus cultures undergo marked changes in structure when transferred to a differentiating medium containing benzyladenine,which is known to induce organ differentiation in this system.Cells within meristematic regions formed in the callus have small size,large nucleus,some of which contain intranuclear inclusions,with both prominent and vacuolated nucleolus and numerous small vacuoles distributed around the periphery of the cell or dispersed throughout the cytoplasm.In many cases,the presence of cytoplasmic and membranous protrusions was observed in the vacuoles,some of which is in close association with dilated endoplasmic reticulum.Multiple and double membrane-bound concentric structures derived from endoplasmic reticulum were often observed as well.Dictyosomes and their associated vesicles are common,sometimes appear in groups.The cytooplasm contains ribosomes,which are occasionally seen as clusters of polysomes.Therefore,cells within meristematic regions are similar to the vacuolating and dividing cells of meristem.Another distinctive feature of these cells is the appearance of plasmalemma invaginations.They increase the surface area of the plasmalemma of the cell and may facilatate the absorption of the digested contents of neighboring crushed callus cells.It is concluded that these ultrastructural events reflect some morphological changes which precede organ differentiation in the cultured callus of Stevia rebaudiana.